This invention relates generally to the field of nuclear medicine and, more particularly to, radiolabeled agents for imaging tumors, methods of use of said agents, and kits for preparing imaging agents.
Cancer remains a major health problem among humans. In the United States colorectal cancer is the second most common cancer in women after breast cancer, and the third most common cancer in men after prostate and lung cancer. While the colorectal cancer incidence rates for white and black males are similar (roughly 61 per 100,000 population), the rate for white females (88 per 100,000 population) is higher: than that for black females (79 per 100,000 population). However, the mortality rate for both black males and black females is higher than their white counterparts, suggesting that late diagnosis may contribute to the mortality rate. Moreover, in that the incidence and mortality rates related to this disease continue to rise, the need for early diagnosis is of undisputed importance for management of the cancer. (Scottenfeld, D., Cancer of the Colon, Rectum and Anus, Chapter 3, Epidemiology, Cohen, A. (Ed.), McGraw Hill, Inc., New York, 1993, pp. 11-24). If the diagnosis is made while the cancer is still silent or occult, the cancer may be treated with conservative surgery, with or without additive, radio- or chemotherapy.
During the past 15 years, advances have been made in diagnostic imaging of tumors. The introduction of computerized tomography (CT) by Hounsfield in the early 1970""s, enabled more accurate detection with higher spatial resolution for small foci of cancer than previous x-ray technology. Similarly, magnetic resonance imaging (MRI) improved the diagnostic capabilities for detecting bone malignancies.
Gamma cameras have generally been used for diagnostic purposes because they are considerably less expensive than either CT or MRI scanners, and the scintigraphic studies are more cost effective than the CT or MRI imaging. Gamma cameras enable a patient""s entire body to be scanned at one time and in a relatively short period, allowing a single diagnostic test to be utilized for all imaging needs. However, while nuclear medicine can facilitate the diagnosis of cancer, there remains a need for better imaging techniques and more specific imaging agents for detecting tumors, particularly those that have metastasized.
Recently, CT, MRI and ultrasound imaging have been criticized for the detection of colorectal adenocarcinoma. (Beutow, P. C. et al., Radiographics 15: 127-146, 1995). The manner of routine screening for this disease remains controversial, and it is generally agreed that CT should not be used routinely to stage colorectal carcinoma because of its low accuracy. In that the five year survival rate patients with colorectal cancer is only 5%, and that this rate has not changed for the past 40 years, it is evident that the early detection of small cancers and pre-malignant adenomas is necessary to improve the survival rate. For early detection to be practical, new non-invasive methods of detection of this disease are necessary.
Biomolecules specific for tumor antigens, when labeled with gamma emitting radionuclides, can provide an efficient means of detecting lesions. Oncoscint, an In-111 labeled anti-CEA and anti-TAG-72 antibody, has been utilized as a tumor imaging agent. However, this agent has had only limited success because of relatively low sensitivity (40%) and specificity (50%) (Beatty, J. D. et al., Cancer of the Colon, Rectum and Anus, Radioimmunoscintigraphy, Chapter 77, Cohen, A. (Ed.), McGraw Hill, Inc., New York, 1993, pp. 753-767; John, T. M. et al., Proc. Annu. Mtg. Am. Soc. Clin. Oncol. 13: A275, 1994), and because it induces an immunologic reaction in 30% of the patients. (Corman, M. L. et al., Diseases of the Colon and Rectum 37: 129-137, 1994). The imaging agent is also of limited because of its high liver uptake which precludes its use for detecting liver metastases. (Abdel-Nabvi, H. H. et al., Targeted Diag. and Therapy 6: 78-88, 1992). Agents with improved specificity and sensitivity, and ones that can be used reliably to detect liver metastases would be widely accepted by clinicians.
Radiolabeled receptor peptides specific for imaging tumors, abscesses and vascular thrombi are particularly attractive for use as radioimaging agents because they are smaller in size, easier to produce, and cleared more rapidly from the blood than other receptor specific larger molecules, such as radiolabeled monoclonal antibodies. In-111-Octreotide, a radiolabeled somatostatin analog, has been used to image endocrine tumors. However, it does not appear that this imaging agent is as sensitive as an I-123 labeled vasoactive intestinal peptide (VIP). Only four positive scans were obtained in 17 patients with colonic adenocarcinoma using In-111-Octreotide, as compared to 17 out of 17 positive scans with I-123 labeled VIP. (Virgolini, I. et al., New Eng. J. of Med. 33: 1116-21, 1994).
VIP is a 28 amino acid neuroendocrine mediator that has been detected on the cell surface membrane of intestinal epithelial cells (Virgolini, I. et al., Cancer Res. 54: 690-700, 1994; Blum, A. M. et al., J. Neuroimmunol. 39:101-8, 1992), lungs (Couvineau, A. et al., J. Clin. Endocrinol., Metab. 61: 50-55, 1985), and various tumor cells, including colonic adenocarcinomas (el Battari, A. et al., J. Biol. Chem. 263: 17685-9, 1988), pancreatic carcinomas (Svoboda, M. et al., Eur. J. Biochem 176: 707-13, 1988) and carcinoids (Virgolini, I. et al., Cancer Res. 54: 690-700, 1994). The amino acid sequence for VIP, which is identical for porcine, bovine, dog and human, is His Ser Asp Ala Val Phe Thr Asp Asn Tyr Thr Arg Leu Arg Lys Gln Met Ala Val Lys Lys Tyr Leu Asn Ser Ile Leu Asn (SEQ ID NO: 1). It was initially isolated from porcine intestine more than 25 years ago. (Said, et al., Science 69: 1217-1218, 1970).
When labeled with I-123, VIP has been shown to be effective in imaging colorectal adenocarcinoma (100%), including in patients with liver metastases (83%). However, the lung uptake of this agent is very high: approximately 25% at four hours post-injection and 10% at 24 hours post-injection. It is unclear at present whether or not the high lung uptake is related to lung receptor density or to iodination of the two tyrosine residues (residues 10 and 22 of VIP). Accordingly, iodinated VIP adversely affects imaging of lung metastases (66%).
Further, although I-123 is widely used for scintigraphic imaging, and may be a logical choice to label a tyrosine containing peptide, it is a cyclotron produced radionuclide with a relatively short half-life (13.3 hours). It is therefore expensive to produce, and must be ordered within 24 hours of its intended use to prevent excessive radioactivity decay. For these reasons I-123 has not been widely utilized for clinical applications.
Conversely, Technetium-99m (xe2x80x9cTc-99mxe2x80x9d), is produced on a generator from its parent radionuclide Molydenum-99 (xe2x80x9cMo-99xe2x80x9d), which has a substantially longer half-life (2.8 days). Mo99, bound on solid matrix in several mCi quantities, can be purchased once a week. The Tc-99m is washed off with sterile isotonic saline once or twice a day as needed. Tc-99m decays within a six hour half-life with the emission of 140 keV gamma rays. The shorter half-life permits rapid decay of the radioactivity, minimizes the radiation dose to normal organs, and eliminates the need for patient hospitalization. Its gamma ray energy also allows efficient detection by gamma cameras. Further, because of the generator system, the radionuclide is relatively inexpensive. For these reasons Tc-99m has become the radionuclide of choice in nearly 90% of the clinical nuclear medicine applications.
It has now been found that a modification of the peptide sequence His Ser Asp Ala Val Phe Thr Asp Asn Tyr Thr Arg Leu Arg Lys Gln Met Ala Val Lys Lys Tyr Leu Asn Ser Ile Leu Asn (SEQ ID NO: 1) can be used to image tumors, particularly colonic adenocarcinomas, liver metastases, pancreatic carcinomas, and carcinoids.
The present invention provides a composition useful as a radiodiagnostic agent for imaging tumors, particularly endocrine tumors, liver metastases, and carcinoids in mammals, a method and a kit for the preparation of the tumor imaging agent, a reagent for radiolabeling the imaging agent, and a method of use for the tumor imaging agent. Specifically, the tumor imaging agent is comprised of a composition containing a tumor specific sequence (TSS) and a radionuclide moiety linked to the TSS via a linker, wherein a radionuclide is complexed to the radiolabeling moiety. In a preferred embodiment, the composition comprises the sequence His Ser Asp Ala Val Phe Thr Asp Asn Tyr Thr Arg Leu Arg Lys Gln Met Ala Val Lys Lys Tyr Leu Asn Ser Ile Leu Asn Xaa Gly Gly (D)Ala Gly (SEQ ID NO: 2).
A second embodiment of the invention provides a reagent for radiolabeling a TSS, comprising four amino acids, selected from the group consisting of glycine and alanine, which can covalently link a selected radionuclide to the amino groups of each amino acid to form an N4 configuration.
Still another embodiment of the invention provides a kit for preparing a thrombus imaging agent, which kit comprises a container capable of holding a multiple of vials and reagents. A first vial contains an appropriate quantity of prepared TSS and radionuclide moiety for reacting with a radionuclide. A second vial contains an appropriate buffer,
The invention also provides a method of imaging endocrine tumors in a mammal by obtaining in vivo gamma scintigraphic images. The method comprises administering an effective diagnostic amount of the thrombus imaging agent to a mammal in need of such imaging and detecting the gamma radiation emitted by the imaging agent localized at the thrombus site within the mammal.